Severer requirements have been continuously imposed on soft magnetic materials. Basic requirements are high saturation magnetization, high magnetic permeability, and low core losses. To meet these requirements, the soft magnetic materials should satisfy the conditions that (1) their magnetostriction constant .lambda.s is as low as .+-.5.times.10.sup.-6, and (2) their crystalline magnetic anisotropy is low. If these two conditions were not met, there would be soft magnetic materials which have no satisfactory basic properties or are not useful at all in some applications.
More particularly, in an application where stresses are applied at all times during operation as in the case of magnetic heads, during manufacture of magnetic cores, typically compressed powder cores, or in an application where stresses are applied to cores at all times, the useful soft magnetic material should have a zero or negative magnetostriction constant .lambda.s, especially of the order of from 0 to -5.times.10.sup.-6.
Known soft magnetic materials of the iron base alloy type include pure iron, silicon steel, Sendust alloys, and amorphous iron base alloys, all of which are characterized by a high saturation magnetic flux density. Among these soft magnetic materials, amorphous iron base alloys have become widespread because of their high saturation magnetic flux density and low iron losses.
However, amorphous iron base alloys can find only limited applications because of their high magnetostriction constant. The amorphous iron base alloys have made little progress in those applications where stresses are applied, for example, magnetic heads, smoothing choke coils, compressed powder cores, and magnetic shields because there arises an essentially serious problem that magnetic properties are substantially deteriorated.
Among the amorphous alloys, however, there are known amorphous cobalt base alloys having a magnetostriction constant of approximately zero. Unfortunately, the cobalt base alloys have a low saturation magnetic flux density and are expensive. They are thus used in only those applications where the material cost is not a predominant factor, for example, such as magnetic heads.
One approach to solve the problems associated with amorphous alloys is an iron-base soft magnetic alloy having a fine crystalline phase as proposed in EPA Publication No. 0 271 657 A2 (Hitachi Metals Co., Ltd., published 22.06.88). This soft magnetic alloy is prepared by first forming an amorphous alloy of the corresponding composition, and then heat treating the alloy so as to develop a fine crystalline phase. This alloy improves over the conventional amorphous iron base alloys. A substantial reduction in saturation magnetostriction constant is especially desirable. Nevertheless, this alloy is still unsatisfactory in some aspects. In particular, it is impossible to manufacture an alloy having a zero or negative magnetostriction constant. Therefore, the alloy cannot be practically used in those applications where stresses are applied, for example, such as magnetic heads. The above-referred publication describes an example in which a magnetostriction constant approaches zero at a boron (B) content of about 5 atom % (e.g., Fe.sub.74 Cu.sub.1 Nb.sub.3 Si.sub.17 B.sub.5 alloy). However, it is generally well known that alloys having a boron content of about 5 atom % are difficult to render amorphous. In addition, the alloy of the above-referred publication is quite low in corrosion resistance which is of basic importance for metallic materials.
Alloys having a fine crystalline phase are prepared by heat treating an amorphous alloy as described above. In turn, the amorphous alloy is prepared by rapid quenching from a melt by a single or double chill roll method. The single and double chill roll methods involves injecting a molten alloy against the surface of a chill roll through a nozzle, thereby rapidly quenching the alloy for forming a thin ribbon or piece of amorphous alloy. Rapid quenching is desirably carried out in a non-oxidizing atmosphere in order to prevent oxidation of the melt.
It is, however, difficult and expensive to strictly maintain a non-oxidizing atmosphere. Therefore, the atmosphere generally used in rapid quenching contains some oxygen so that the melt is somewhat oxidized near the nozzle tip. The oxide of the melt forms a scale which deposits on the nozzle tip. The nozzle is thus blocked as the melt injection is continued, requiring replacement of the nozzle or in some cases, causing breakage of the rapid quenching apparatus. The nozzle blockage becomes a serious problem for mass production requiring continuous injection of an alloy melt for an extended period of time. A highly viscous alloy melt tends to promote nozzle blockage because the melt injection becomes more difficult due to a reduction of nozzle diameter by oxide deposition. The nozzle blockage is detrimental to mass production and cost.
Choke coils, for example, common mode choke coils and normal mode choke coils as noise filters are utilized in smoothing an output of a switching power supply. A choke coil is arranged to allow for passage of AC current flow overlapping DC current flow. The core of the choke coil should have such magnetic properties that its magnetic permeability changes little as the intensity of an applied magnetic field varies, that is, constant magnetic permeability. If squareness ratio (residual magnetic flux density/saturation magnetic flux density, Br/Bs) is high, application of intense pulsative noises causes the operating point to shift to the point of residual magnetization Br, at which magnetic permeability is markedly inferior to that at the operating point originally located at the origin of the B-H loop. Therefore, constant magnetic permeability can be accomplished by increasing the unsaturation area in the B-H hysteresis diagram, or evening out the B-H loop.
One exemplary magnetic core material having high magnetic permeability is an iron base magnetic alloy having fine crystalline particles as disclosed in Japanese Patent Application Kokai No. 142049/1989. This iron base magnetic alloy is prepared by heat treating an amorphous alloy so as to develop fine crystalline particles. According to the disclosure of Kokai, the iron base magnetic alloy is improved in core loss, variation of core loss with time, and permeability and other magnetic properties. Especially noted, it has a saturation magnetostriction constant as low as within .+-.5.times.10.sup.-6. Since this iron base magnetic alloy has high squareness property irrespective of a low saturation magnetostriction constant, it is formed into a core of a common mode choke coil by heat treating the alloy in a magnetic field applied in a direction perpendicular to the magnetic path (the direction of a magnetic flux extending when used as the core), thereby slanting the B-H curve or loop for achieving a low squareness ratio and constant permeability. In order that the magnetic field be applied in a direction perpendicular to the magnetic path, the entire core must be placed in a uniform magnetic field. A large size magnet is then necessary. An extremely larger size magnet is necessary in order to apply a uniform magnetic field over a plurality of cores at the same time. This impractical scale-up results in reduced productivity. Thus the heat treatment in a magnetic field is not amenable to mass production of cores at low cost. Further, although the heat treatment in a magnetic field applied in a direction perpendicular to the magnetic path results in a core having a low squareness ratio, its magnetic permeability can change during use because the applied magnetic field is offset 90.degree. from the magnetization direction of an actual common mode choke coil.